Conventional fractional frequency synthesizers achieve a non-integer loop dividing in average by modulating the loop dividing ratio (NDIV). Most of the conventional implementations use the output of the digital Sigma-Delta Modulator (SDM), that is the NDIV, to modulate the loop dividing ratio. Such conventional implementations may introduce quantization errors in the loop dividing ratio, which can be reflected as phase noise at outputs of the fractional frequency synthesizers. The introduced quantization errors or noise may seriously affect the performance of the frequency synthesizer without special treatment.
In conventional methods of suppressing quantization noise, an additional filter is usually introduced in the loop to achieve suppression of high-frequency quantization noise to improve the performance of the frequency synthesizer. However, the additional filter may affect the phase margin of the overall loop. In many applications where a wide loop bandwidth is required, the noise near the bandwidth is amplified, thereby may degrade the quality of the phase-locked loop output signal, and may affect the performance of frequency synthesizer. In order to reduce such negative influence, the roll-off degree introduced by the additional filter is limited to a first order, which cannot meet the requirements for suppressing quantization noise. Therefore, it is difficult for a conventional fractional frequency synthesizer to add a filter with desirable performance.